Limitations in cancer cure are due to both therapy-related complications such as myelosuppression and disease relapse due in part to cancer stem cells. To shorten the duration of myelosuppression, improved understanding of bone marrow injury and repair mechanisms could identify novel stem cell growth factors. Historically, the search for hematopoietic growth factors has focused on intrinsic signaling mechanisms, but we have sought to identify extrinsic signaling mechanisms that regulate hematopoietic stem cell (HSC) regeneration. Based on a cytokine array of secreted factors from the bone marrow of radioprotected mice bearing deletions of pro-apoptotic genes Bak and Bax, we identified elevated levels of epidermal growth factor (EGF). Our preliminary results show for the first time that HSCs express EGF receptor (EGFR), and activation of EGFR accelerates HSC regeneration after radiation. Since HSCs and leukemia stem cells (LSCs) share common signaling pathways, we screened for EGFR (ErbB1) and ErbB2, 3, and 4 in 10 patient samples with acute myeloid leukemia (AML). We detected ErbB2 overexpression, ligand-independent phosphorylation of ErbB2, and truncated forms of ErbB2 in primary AML cells. Extrapolating from ErbB2+ breast cancer biology, we know that alternate, truncated forms of ErbB2 are functional in breast cancer, and targeted therapies to inhibit ErbB2 signaling has successfully decreased tumor burden in ErbB2+ breast cancer. We expect that modulation of ErbB2 signaling will decrease LSC burden as well. To further investigate these findings, we propose two specific aims: 1. Determine the function of EGFR in regulating HSC regeneration after myelosuppression and 2. Determine to what extent ErbB2 signaling governs LSC self-renewal and sensitivity to chemotherapy. Our approach in Aim 1 is to genetically delete EGFR in Vav+ hematopoietic cells. Following exposure to radiation or chemotherapy with 5-fluorouracil, we will measure hematopoietic stem/progenitor cell content and cellular responses (i.e., cell death, cell cycle, and cell proliferation) in EGFR-deficient mice compared to control mice. We expect that deficiency of EGFR will result in substantial delays of hematopoietic recovery and will determine whether the deletion of PUMA in EGFR-deficient mice is sufficient to rescue HSCs from radiation-induced death. Therefore, EGFR signaling could be a therapeutic target to accelerate stem cell regeneration in cancer patients or those undergoing stem cell transplantation. In Aim 2, our approach is to pharmacologically and genetically modulate ErbB2 signaling and determine how ErbB2 inhibition decreases LSC burden in combination with chemotherapy. Since ErbB2 is a well-established oncogene in breast cancer, we will determine whether ErbB2 enforced activation in HSCs is sufficient to accelerate the onset of AML in a genetically-defined model of AML. In summary, we have strong preliminary evidence that EGFR/ErbB2 signaling is crucial in HSC and LSC maintenance and regeneration. Completion of these aims will further elucidate the mechanisms by which EGFR signaling accelerates HSC regeneration and provide insights into the role of ErbB2 in promoting leukemogenesis.